Ceramic coated fuel particles



1964 L. D. STOUGHTON ETAL 3,121,047

CERAMIC COATED FUEL PARTICLES Original Filed March 16, 1961 lNVENTORS.

LINCOLN D. STOUGHTON JOHN M. BLOCHER, JR. NE'L D. VEIGEL United States atent Ofi 3,121,047 Patented Feb. 11, 1964 ice 3,121,047 CERAMIC COATED FUEL PARTICLES Lincoln D. Stoughton, Chatham, N.J., and John M.

Blocher, Jr., and Neil D. Veigel, Columbus, Ohio, as-

signors to the United States of America as represented by the United States Atomic Energy Commission )riginal application Mar. 16, 1961, Ser. No. 96,338. Di-

vided and this application May 11, 1962, Ser. No.

Claims. Cl. 17669) This invention relates to ceramic coated fissionable fuel articles and in particular to ceramic coated fissionable uel particles in which a fission product sump inter-layer l provided.

This application is a division of application Serial lumber 96,338, filed on March 16, 1961.

The dispersion of fissionable fuel particles in a matrix ffers a promising solution to several high-temperature uel element problems provided some way of controlling ssion-product release, corrosion, and fuel migration can e found. A matrix of this type would be suitable for xample as the spherical fuel elements in a pebble bed as-cooled reactor such as described in patent application, erial No. 767,242, filed October 14, 1958, in the names f Robinson and Stoughton. The core of such a heliumooled reactor consists of a bed of fueled-graphite spheres. he use of surface coatings for the spheres themselves, ether than coating for the individual particles has been nvestigated, and while such an arrangement does have adantages, there are certain disadvantages such as the heavy ission product release which would occur should a coatng rupture as compared to the coating on a single particle, nd further the greater risk of such ruptures since the utside coatings are themselves without surface protecion as are the particle coatings surrounded by matrix.

As a result, it has become apparent that individually oated fission fuel particles, such as U0 coated with U 0 would have certain distinct advantages over the pherical outer coatings provided some way could be ound to provide highly impervious particle coatings apable of uniform application and avoiding the excess resses which can be expected to result because of fission roduct accumulation within the tight coating without rovision for absorbing such products and thereby reeving such stresses. In such a tightly fitting impermeble coating, certain of the fission products which are )und in the U0 particle are gaseous (i.e., such as xenon, rypton, etc.) and with little free volume inside the parcle coating into which these gases could expand gas ressure build-up which could rupture the coatings will mit the useful life of the coated fuel particle. Accord lg to some calculations made for this purpose, it has een found that for a U0 particle of 95% theoretical ansity, for 50% release of all gaseous fission products om the U0 crystals in a fuel element exposed to 000 kw.h. irradiation, and assuming that the only volume vailable to store the fission gases is the pore volume of 1c U0 the gas pressure would be about 23,500 p.s.i.

This invention concerns improved coated particles and aherical fuel elements to provide the high degree of imerviousness and reliability required for the utility hereibCfOl'C described. In the coated particle of this invenon, the coating is applied to form a porous inter-layer t contact with the fissionable material and the impervious .iter-layer for containing all fission products. The new inner layer produces several particular advantages. he pore volume of the porous inner layer will act as a iservoir for fission product gases and increase the useful he of the coated fuel particle. Further, if the porous H161 layer is thicker than the recoil range of fission agments in the coating material, then the outermost impermeable coating will not be damaged by having to absorb the kinetic energy of fission fragments recoiling from the surface of the U0 particle. Another advan tage lies in reducing the critical effect of any differences in coefiicients of thermal expansion of the coating and particle material which may exist. For example, a higher coefficient for the fuel particle could result in rupture of the coating when the particle is heated above its fabrication temperature. The porous inner layer could absorb the differences in thermal expansion, which would otherwise crack a tight dense coating applied directly to the fuel particle, thus permitting a higher operating temperature for the coated particle. Still another advantage is that fission product diffusion rates from the coated particle is reduced by presence of the porous inner layer thicker than the fission fragment recoil length since no fission fragments will come to rest in the outermost impermeable layer of the coating. Work on other materials which are normally impermeable to fission products [i.e., metals) has shown that when fission fragments recoil directly into the material, they diffuse through the material at a much faster rate than fission products which come to rest before entering the material. Thus, the porous inner layer will prevent fission products entering the coating directly by recoil and thereby reduce the diffusion rate of fission products from the coated fuel particle. Another and perhaps the most important advantage of the porous inner layer is in effect the provision of a frangible layer which may be crushed under the impact of the expanding U0 which swells under radiation thereby reducing the stresses which would be imposed on the outer layer by the swelling of the U0 in the absence of the porous inner layer.

A known method of depositing dense impermeable tightly inherent ceramic A1 0 coatings on small U0 particles is to do this in a fluidized bed wherein aluminum chloride is hydrolized in the presence of hydrogen at 1000 C. in such a manner that alpha-phase A1 0 is formed. This technique is modified to obtain the porous inner layer, including the initial reduction which is conducted at a somewhat lower temperature to form the porous layer of the A1 0 It is thus a first object of this invention to provide a ceramic coated particle of fissionable fuel with a fission sump frangible inner layer.

It is a further object to provide a U0 particle with a ceramic coating and a porous ceramic layer in between to act as a sump for gaseous fission products.

It is still another object to provide fission fuel material consisting of a matrix containing dispersed U0 particles each provided with an impervious ceramic coating to contain fission products and a porous inner layer for each particle to act as a reservoir for fission product gases.

Other objects and advantages of this invention will hereinafter become more apparent from the following discussion and with reference to the drawings in which:

FIG. I is a schematic diagram of apparatus for producing A1 0 coated U0 particles in accordance with this invention;

FIG. 2 is a photomicrograph of a typical graphite matrix of spherical U0 with 20 micron coatings of A1 0 at x.

FIG. 3 is a photomicrograph of a U0 particle in FIG. 2 at 500x with a 20 micron coating of A1 0 all alpha phase; and

FIG. 4 is a photomicrograph of U0 particles at 50X with an outer coating of impervious A1 0 and a porous inner layer of A1 0 It has been found that for use in a nuclear reactor, such as the pebble bed reactor noted above, fuel elements consisting of a carbon or graphite matrix with a dispersion of U particles individually coated with a tight impermeable ceramic coating such as A1203 afforded many advantages previously enumerated. The fluidized bed process of preparing these coated particles had been developed and it was thus made possible to prepare such particles in large numbers with very uniform and excellent results. Further, this technique, using chemical vapor deposition, which is defined as the formation of a solid deposit by chemical reaction of vapors at the heated surface, was ideally suitable since dense coatings could be produced which could be expected to provide good fission-product retention and prevent contact between fuel and corrosive environments.

However, in addition to the other drawbacks which presented itself in the preparation and use of this type of coated fuel particle was that the coeflicient of expansion of U0; is somewhat greater than that of A1 0 As a result, the tight impermeable coating would be placed in tension as the particles are heated above the coatingdeposition temperature. These stresses are intensified by swelling of the U0 after undergoing radiation and by the containment within the tight coating of fission product gases released by the U0 Referring to FIG. 1, for a brief description of the apparatus for the deposition of A1 0 on U0 particles, there is shown a reactor comprising a quartz tube with a conical bottom 12 to support the bed of U0; powder 14. Reactor 10 is maintained at proper temperature by electrical resistance heating elements 16. Fluidizing gas is supplied to reactor 10 at the bottom from an AlCl vaporizer 18 which is heated by electrical resistance elements 22. Hydrogen is supplied to vaporizer 18 from line 19 to become partially saturated with aluminum chloride, with additional hydrogen added from line 24 as needed. A separate stream of hydrogen may be passed through a water vaporizer 26 from line 27 and through an axial tube 28 which terminates in the lower part of fluidized bed 14 where mixing of the reactants occurs at the desired temperature. The gaseous results of the reaction leave reactor 10 at the top through pipe 32 and pass through successive dust traps 34 and 36, filters 38 and 42, and a scrubber 44 which is supplied with water from a reservoir 46. The remaining gases are vented to atmosphere through a pipe 48. In FIGS. 2 and 3 photomicrographs illustrate typical A1 0 coated particles previously produced by this technique.

In operating the apparatus of FIG. 1 to obtain the porous inner layer described above, the reaction is maintained at 200 to about 900 C. in the lower part of the bed where the mixing of the reactants occurs for sufficient time for the porous coating to reach desired thickness and then the temperature is raised to from about 900" to 1400 to continue the reaction with the production of the alpha phase A1 0 Particles in the range of 50 to 400 micron size were found to be suitable for carrying out this invention.

In carrying out this vapor deposition process as described above, the particles in the fluidized bed circulate from the top of the bed to the bottom. Since the particles in the bottom of the bed are cooled somewhat by the incoming gas, a truly isothermal reactor in connection with carrying out the inventive process especially but not only in the first and lower temperature of the process is not desirable. Thus, temperature cycling of the reactor within the ranges set forth above produce very desirable uniform high quality coatings in both the inner and outer layers in accordance with this invention.

4, The chemical reaction occurring in reactor 10 is as follows:

Al Cl +3H O Al O +6HC1 In one example of this invention, a batch of to U0 powder was coated in fluidized bed 14 with 20 micron coatings of A1 0 The reaction was carried out as described above, with hydrogen, partially saturated with aluminum chloride, constituting the major portion of the fiuidizing gas. A 100-g. bed of the U0 powder was placed in the 1 inch diameter reactor 10. The reaction was conducted at 750 C. for 6 hours to build up the porous layer of the A1 0 on the particles and then reactor 10 was vented and the temperature raised to 1000 C. where the reaction was continued for 12 hours to deposit the impermeable alpha phase Al O In FIG. 4 a coated U0 particle with a porous inner layer is shown. The alpha phase A1 0 in the outer layer is shown to be very dense. The inner layer is porous and thus not as dense as the outer layer.

Reactor 10 would be limited to a 5 inch diameter when fully enriched U0 powder is used to avoid criticality problems. However, for less than fully enriched powder, large reactors may be used.

The coated U0 particles would then be distributed by known techniques in a graphite matrix formed into spherical elements and then, if desired, used in a pebble bed reactor.

Tests on coated U0 particles provided with the porous A1 0 sump show a sharp extension in their life during exposure to intense radioactivity. Particles of this type showed remarkable resistance to thermal stresses imposed by large temperature cycling and certain impact tests.

It is thus seen that there has been provided improved U0 coated particles for preventing release of gaseous fission products and an improved fuel assembly based on such coated particles.

We claim:

1. An article of fissionable material for the self containment of fission products comprising a particle of U0 a first coating on said particle of porous A1 0 and a second coating surrounding said first coating of dense, alpha phase A1 0 2. The article of claim 1 in which the U0 particle is of 50 to 400 micron size.

3. A fuel element for a nuclear fission reactor comprising a plurality of U0 particles dispersed in a matrix of carbon, each of said particles having a first coating of porous A1 0 and a second coating surrounding said first coating of dense, alpha phase A1 0 4. The fuel element of claim 3 in which said carbon matrix is a graphite sphere.

5. The fuel element of claim 4 in which the U0 particles are of 50 to 400 micron size.

References Cited in the file of this patent FOREIGN PATENTS Great Britain Mar. 30, 1960 OTHER REFERENCES 

1. AN ARTICLE OF FISSIONABLE MATERIAL FOR THE SELF CONTAINMENT OF FISSIONPRODUCTS COMPRISING A PARTICLE OF UO2, A FIRST COATING ON SAID PARTICLE OF POROUS AL2O3, AND A SECOND COATING SURRONDING SAID FIRST COATING OF DENSE, ALPHA PHASE AL2O3. 